Zinc oxide (ZnO) nanoparticles have been traditionally used to block sunlight in a wide spectrum range. Due to superior semiconducting property with a very large band gap energy, as well as biocompatibility, piezoelectricity, fluorescence and optical conductivity, applications in various fields, including solar devices, biochips, gas sensors, catalysts and electronic devices, are expected.
ZnO nanoparticles are prepared by a gas phase synthesis process wherein gaseous Zn is reacted with oxygen, by a coprecipitation process wherein a Zn precursor is dissolved in water, precipitated as zinc carbonate, and then heat-treated to obtain ZnO nanoparticles, or by a solution process wherein a Zn precursor is dissolved and then an alkali salt solution is added to obtain ZnO precipitate.
The gas phase synthesis process is disadvantageous in that control of ZnO particle size is difficult, preparation of ZnO particles having size of tens of nanometers is difficult, and process and facility to conduct the relevant gas phase reaction at high temperature are complicated. Thus, it is not suited for large-scale production.
The coprecipitation process is disadvantageous in that, since ZnO is prepared by heat treatment, ZnO aggregates produced by sintering during the heat treatment should be pulverized by a post-treatment process. Also, it is difficult to prepare ZnO nanoparticle with uniform shape and narrow particle size distribution of tens of nanometers.
The solution process requires addition of a dispersant to control the size of ZnO nanoparticles. Even when the dispersant or other additive is used, ZnO particles of an order of hundreds of nanometers are prepared, and needle-shaped particles are obtained rather than spherical ones. Further, an expensive organozinc compound is used as a Zn precursor. Although the synthesis proceeds at relatively low temperature (300° C. or lower) as compared to the gas phase synthesis or coprecipitation process, a long time is required until the reaction is completed.
Zinc oxide (ZnO) nanofluid wherein ZnO nanoparticles are dispersed in a fluid has very high thermal conductivity as compared to a fluid without containing the nanoparticles. Hence, researches are increasing for utilizing the property with industrial purposes. The nanofluid having improved thermal conductivity may be used to improve thermal efficiency of heat exchangers, automobile engines, or the like, and therefore is widely applicable in the fields of electricity, electronics, machinery and others.
The technical problems in the preparation of nanofluid are how to keep the fluid stably dispersed for a long period of time and how to produce the nanofluid with good dispersion stability in large scale via a simple process.
At present, commercially available nanoparticles are mixed with a medium such as water or alcohol, dispersed for 30 to 40 hours using ultrasonic wave, and then mixed for 30 to 40 hours after adding a solution of benzonite, phosphate, nitrate, etc. in ethylene glycol to prepare nanofluid (Korean Patent Publication No. 2007-0096505), or commercially available nanoparticles are dispersed in liquid solvent, physically pulverized using a bead mill or high-pressure homogenizer, surface-modified, passed through an ultrafiltration membrane, and then dispersed in oil after removing water to prepare nanofluid (Korean Patent Publication No. 2008-0038625).
However, these processes are disadvantageous since each step of the processes requires a long time of 30 to 40 hours or the process of pulverization, high-pressure homogenization or filtration is complicated and requires expensive equipments, which makes them inapplicable to large-scale production. Further, a new preparation process has to be designed for a different dispersion medium.